Biljana Bobić

Corrosion behaviour of thixoformed and heat-treated ZA27 alloys in NaCl solution

Abstract The influence of corrosion on the microstructure of thixoformed and heat-treated ZA27 alloys was investigated. The microstructure of ZA27 alloy was affected by heat treatment. The process of electrochemical corrosion occurs in both ZA27 alloys through the area of η phase. According to the results of immersion test and electrochemical measurements, the corrosion rate of the thixoformed ZA27 alloy is at least 50% lower than that of the thixoformed and thermally processed alloy. This indicates the unfavourable influence of applied heat treatment (T4 regime) on the corrosion resistance of the thixoformed ZA27 alloy.

Microstructure and mechanical properties of Zn25Al3Cu based composites with large Al2O3 particles at room and elevated temperatures

Abstract Microstructures and compressive properties of Zn25Al3Cu alloy and Zn25Al3Cu/Al2O3 particulate composites with large reinforcing particles (250 μm) were examined. The composites were obtained by the compocasting technique through infiltration of 3, 8 and 16 wt.% Al2O3 particles into the semi-solid metal matrix. The influence of temperature in the range from 70 to 170°C on compressive yield strength of the matrix alloy (as-cast and thixocast Zn25Al3Cu alloy) and the composites was investigated. Above 70°C compressive yield strength of all materials decreases, but the rate of decrease is different for the matrix alloy (as-cast and thixocast) and composites. It was found that the abrupt decrease in compressive yield strength of the matrix alloy occurred at temperatures higher than 70°C, whereas composites retained relatively high values of compressive yield strength till the end of the testing temperature range regardless of the amount of reinforcing particles.

Corrosion influence on surface appearance and microstructure of compo cast ZA27/SiCp composites in sodium chloride solution

Abstract The influence of corrosion on the surface appearance and microstructure of particulate ZA27/SiCp composites was examined after 30 d immersion in a sodium chloride solution with the access of atmospheric oxygen. The composites with different contents of SiC micro-particles were synthesized via compo casting. Microstructural studies by means of optical microscopy (OM) and scanning electron microscopy (SEM) showed that corrosion occurred in the composite matrices, preferentially in regions of the η phase, rich in zinc. The corrosion processes did not affect the silicon carbide particles incorporated in the matrix alloy. According to the results of electrochemical polarization measurements, an increase in the content of SiC particles in the composite matrice has led to the lower corrosion resistance in the composites.

Artificial aging of thixocast ZA27 alloy and particulate ZA27/SiCp composites

Abstract Thixocast ZA27 alloy and particulate ZA27/SiCp composites were subjected to artificial aging at 80, 120 and 160°C (T5 regime). Composites with 5 and 10 vol.% SiC particles were produced via the compocasting method. The influence of the aging was investigated using different techniques. Differential scanning calorimetry was performed to reveal the presence of phases created in the thixocast ZA27 alloy and the composites. Microstructures and fracture behavior were examined by means of optical microscopy, scanning electron microscopy and X-ray diffraction. Aging processes in the matrix alloy and composites were followed using hardness measurements. A decrease in the hardness values was noticed with the increase in the aging temperature. At higher temperatures diffusion of zinc atoms from supersaturated phases in the matrix alloy was enhanced, which resulted in faster changes in the hardness. Aging processes in the composites were significantly accelerated compared to the matrix alloy, due to the presence of particulate reinforcements. The maximum in hardness was achieved for shorter time in the ZA27/SiCp composite with higher volume fraction of SiC particles.

Influence of welding parameters on pit initiation and pit growth in welded joints of X5CrNi18-10 stainless steel

The influence of welding parameters on the resistance to pitting corrosion in welded joints of X5CrNi18-10 stainless steel was studied. Welding was performed by a TIG procedure in a shielding gas containing different amounts of nitrogen. The welding speed, the deposition rate of the filler wire, and the shielding gas flow rate were constant, which allowed the nitrogen content in the weld metal to be directly dependent on the nitrogen content in the shielding gas. It was shown that welding current and arc energy had a significantly lower influence on the resistance to pit formation and pit growth, than the nitrogen content in the shielding gas. Pitting corrosion testing was conducted using the potentiodynamic polarization method. The resistance to pit formation increases with the increase in nitrogen content in the weld metal, while the resistance to pit growth decreases. The microstructure of the weld metal becomes more homogeneous in the presence of nitrogen and the distribution of chromium more uniform, which leads to the increased resistance to pit formation. Also, due to the formation of ammonia ions, the pH value of the solution within the pit is higher than in the bulk solution, which also increases the resistance to pit formation.

Corrosion resistance of metalized layers on steel parts in ventilation mill

Corrosion behavior of metalized layers, obtained by Plasma Transferred Arc ( PTA ) process and by H igh-Velocity Oxygen Fuel (HVOF) process with the purpose to improve the wear resistance of vital parts of ventilation mill in a thermal power plant, has been tested. The test is performed using three electrochemical techniques, in a solution containing chloride and sulfate ions. It is shown that the steel surface (base metal) dissolves uniformly, without pitting or other forms of local dissolution. Morphology of metalized layers surface indicates that dissolution is non-uniform, but it still can be considered as general corrosion. The corrosion rate of base metal and metalized layer obtained by PTA process is rather low, while the corrosion rate of the metalized layer obtained by HVOF process is much higher. Also, the difference in corrosion potentials between the base metal and the HVOF layer is pretty high but slightly less than maximum allowed difference (prescribed by the standard), to avoid excessive galvanic corrosion. The values of corrosion rate obtained by different electrochemical techniques are in excellent agreement.

Testing of intergranular and pitting corrosion in sensitized welded joints of austenitic stainless steel

Pitting corrosion resistance and intergranular corrosion of the austenitic stainless steel X5Cr Ni18-10 were tested on the base metal, heat affected zone and weld metal. Testing of pitting corrosion was performed by the potentiodynamic polarization method, while testing of intergranular corrosion was performed by the method of electrochemical potentiokinetic reactivation with double loop. The base metal was completely resistant to intergranular corrosion, while the heat affected zone showed a slight susceptibility to intergranular corrosion. Indicators of pitting corrosion resistance for the weld metal and the base metal were very similar, but their values are significantly higher than the values for the heat affected zone. This was caused by reduction of the chromium concentration in the grain boundary areas in the heat affected zone, even though the carbon content in the examined stainless steel is low (0.04 wt. % C).

Testing of susceptibility to intergranular corrosion in welded joints of austenitic stainless steel

The susceptibility to intergranular corrosion of welded joints of austenitic stainless steel X5CrNi18-10 was examined. The testing was performed by the electrochemical potentiokinetic reactivation method with single loop (EPR) and double loop (DL EPR). Also, the susceptibility to intergranular corrosion of stainless steels after senzibilization heat treatment (675 °C/1h) was examined. At given welding parameters (current intensity, the thickness of welded samples of stainless steel, etc.) the base metal is completely resistant to intergranular corrosion, while the heat affected zone shows a relatively low susceptibility to intergranular corrosion. After senzibilization heat treatment the stainless steel showed something higher susceptibility to intergranular corrosion. The advantages of applied electrochemical methods in relation to the classical chemical methods for intergranular corrosion testing were analyzed. DL EPR and EPR methods provide quantitative data on the sensitization degree to intergranular corrosion (in welded joints) and also to intergranular stress corrosion cracking.

Determination of susceptibility to intergranular corrosion of stainless steels type X5CrNi18-10 in field

In this paper, the DL EPR method (electrochemical potentiokinetic reactivation with double loop) was modified and used to study the susceptibility to intergranular corrosion and stress corrosion cracking of a stainless steel type X5CrNi18-10. The tests were performed in a special electrochemical cell, with the electrolyte in the gel form. Modified DL EPR method is characterized by simple and high accuracy measurements as well as repeatability of the test results. The indicator of susceptibility to intergranular corrosion (Qr/Qp)GBA obtained by modified DL EPR method is in a very good agreement with the same indicator obtained by standard DL EPR method. The modified DL EPR method is quantitative and highly selective method. Small differences in the susceptibility of the stainless steel type CrNi18-10 to intergranular corrosion and stress corrosion cracking can be determined. Test results can be obtained in a short time. The cost of tests performed by modified DL EPR method is much lower than the cost of tests by conventional chemical methods. Modified DL EPR method can be applied in the field on the stainless steels constructions.

Mechanisms of hydrogen evolution on chromium

In this work, the mechanisms of hydrogen evolution on the metal chromium in a solution of 0.1 M Na2SO4 + H2SO4, pH 1 to pH 7 were analysed. In the pH < 3 range, hydrogen is evolved according to three different mechanisms, depending on the experimental conditions: electrochemical evolution of H2 by reaction of H ions according to the Volmer-Heyrovsky mechanism, on the bare chromium surface, during chemical dissolution of chromium by the Kolotyrkin mechanism, which does not depend on the electrode potential, and electrochemical hydrogen evolution by the Volmer-Heyrovsky mechanism, but on the passive chromium. At pH values greater than 3, the fourth mechanism of hydrogen evolution is observed, that is electrochemical reaction of water molecules on the surface of oxide covered chromium surface.